Cellular radio with microcellular/macrocellular handoff

ABSTRACT

A cellular radio system comprises a plurality of basic stations each having a radio transceiver serving a cell of the system. A mobile station of the invention has a radio transceiver for communicating with one or other of the base stations, as well as long term averaging units (LTA 0 , LTA 1 , LTA N ) for time averaging handover criterion (HOC) measurements over a first averaging period. The mobile station also includes a summer (S N ) for applying a first hysteresis margin to the time averaged HOC measurements to provide a first handover indicator. The mobile station also includes short term averaging units (STA 0 , STA 1 , STA N ) for time averaging HOC measurements over a second averaging period which is relatively short compared to the first averaging period, as well as a summer (S W ) for applying a second hysteresis margin, which is relatively large compared to the first hysteresis margin, to the time averaged HOC measurements to provide a second handover indicator. The mobile station further includes a gate (OR 1 ) for assessing handover requirements of the mobile station on the basis of the first and second handover indicators, whereby on the one hand relatively gradual variations in HOC can be assessed and on the other hand relatively sudden variations in HOC can also be assessed.

BACKGROUND

I. Field of the Invention

The present invention relates to cellular radio, and in particular,although not exclusively, to a cellular radio system having overlappingmacrocells and microcells.

II. Related Art and Other Considerations

A conventional cellular radio system has a number of radio base stationseach serving a respective radio coverage area or cell, with which amobile radio station can communicate over a radio link. As a mobilestation moves from one cell to the next, the communication link istransferred from the present base station to the next base station usinga procedure known as hand-over or hand-off. The need for hand-over isusually determined on the basis of one or more criteria. Commonly usedcriteria are:

1) received signal strength indication (RSSI) of the mobile at the basestation, or base stations at a mobile station,

2) relative distance measurement of the mobile from the two closest basestations,

3) level of interference from the nearest base station operating on thesame frequency, and

4) in digital systems, bit error rate (BER).

The cells of conventional cellular radio systems are relatively large,typically several kilometers across, and this allows time for data to beacquired for even a fast moving mobile station and a decision made onthe basis of trends in that data. Recently there have been moves towardshaving cellular radio systems with relatively small cells of up to a fewkilometers in diameter, and typically less than one kilometer indiameter, and these are often referred to as "microcells" while therelatively large, sometimes called wide area cells are often referred toas "macrocells". These terms do not indicate absolute size limitationsbut rather reflect the relative size of these two cell types.Microcellular radio systems should provide a better frequency re-use andhence greater user density. Proposals for microcellular systems suggesttheir application to road network, for example motorways where highspeed mobile stations will pass through a microcell very quickly. Thismeans that the time available for measurement of a handover criterion,e.g. RSSI, BER or interference level, for use in a hand-over decision islimited. Furthermore, the radio coverage of microcells is subject tolarge variations in signal strength over short distances relatively, forexample in an urban environment.

Taking RSSI as an example of a possible handover criterion in thehandover decision or cell reselection for an idle mobile, its use inconventional cellular radio systems is complicated by the variation ofRSSI, caused by factors other than pathloss due to changing distance ofthe mobile from a base station. Considering a simplified situation, seeinset sketch in FIG. 1, where a mobile station MS moves between twocells served respectively by base stations BS1 and BS2, the receivedsignal strength of the two base stations at the mobile station variesnot only with distance, i.e. diminishes due to path loss, but alsovaries with fast (Rayleigh) fading and shadowing, see graph in FIG. 1.In this simplified example the `ideal` point for handover is half waybetween the base station where the RSSI for both base stations is thesame--where their pathloss curves intersect. However the effects offading and shadowing lead to uncertainty of the level of RSSI and topathloss, hence render determination of the correct handover point moredifficult. However by time-averaging the RSSIs the effect of fading andshadowing can to some extent be overcome. This time averaging doesintroduce a delay, referred to as the time averaging delay, into thehandover/cell reselection process. The situation is further complicated,however, because if handover occurs where the averaged values for RSSIare the same there is a significant probability that variation in themomentary levels will be sufficient to trigger the system to make anunnecessary handover back to the original base station. To reduce theprobability of such unnecessary handover occuring a hysterasis quantityis introduced which, in effect offsets the RSSI of the current basestation relative to the RSSI of destination base station. A simplifiedexample of this is shown in FIG. 2, in relation to the idealisedsituation of FIG. 1, in FIG. 2 the path loss curve for BS1 is shownoffset by a hysteresis element H, with the result that the intersection(handover) point with the path loss curve (2) for BS2 is also offset, inpositional terms towards BS2.

The two handover parameters, Averaging period (T) and Hysteris margin H,are related to the two handover qualities frequency of unnecessaryhandover, and handover delay D as shown in FIG. 3. The standarddeviation (Sσ) of RSSIs varies with the Averaging period T; thehysteresis margin H is a function of Sσ, and frequency of probability ofunnecessary handover Pu; and Handover delay D is the sum of theAveraging Delay and Hysteris delay. For macrocells of a few kilometersdiameter where the Handover Delay in distance terms is of the order ofseveral hundred meters e.g. 700-800 meters, the Averaging period T andHysteris margin H could typically be T=10 secs and H=7 dB. Suchcriterion can provide satisfactory results in a macrocell environment,however if applied to a microcell context problems occur.

In a microcellular environment large variations of signal level, andhence RSSI of a base station at a mobile station, occur over relativelyshort distances, as shown by the example graph in FIG. 4. As indicatedin the inset sketch in FIG. 4, microcells A+B overlap at a roadjunction, and generally provide coverage over only their respectiveroad. When a mobile MS moves from the road served by base station A intothe road served by base station B, as shown in the inset sketch in FIG.4, the mobile MS experiences the variation in signal strength from thebase stations of the graph in FIG. 4. The significant effect occurs asthe mobile turns the corner, as indicated on the graph, where an almoststep function pathloss effect occurs. The present invention is concernedwith a cellular radio system which seeks to cope with such microcellularsituations while still maintaining the quality of handover control inmacrocellular situations.

SUMMARY

According to a first aspect the present invention provides a cellularradio system comprising: a plurality of base stations each having aradio transceiver serving a cell of the system; a mobile station havinga radio transceiver for communicating with one or other of the basestations; and characterized by the system having on the one hand firstmeans for time averaging handover criterion measurements over a firstaveraging period, means for applying a first hysteresis margin to thetime averaged handover criterion measurements to provide a firsthandover indicator, and having on the other hand a second means for timeaveraging handover criterion measurements over a second averaging periodwhich is relatively short compared to the first averaging period, andmeans for applying a second hysteresis margin, which is relatively largecompared to the first hysteresis margin, to the time averaged handovercriterion measurements to provide a second handover indicator, and meansfor assessing handover requirements of the mobile station on the basisof the first and second handover indicators; whereby on the one handrelatively gradual variations in handover criterion can be assessed andon the other hand relatively sudden variations in handover criterion canalso be assessed.

Preferably the first averaging period is several times the second timeaveraging period, in particular of the order of 20 times. Preferably thefirst hysteresis margin is a significant fraction of the secondhysteresis margin, in particular of the order of half.

A corresponding method is also provided

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described by way ofexample and with reference to the accompanying drawings:

FIG. 1 shows a graph illustrating the variations of received signalstrength at a mobile station (ms) of two base stations (BS1, BS2) atpoints between them, with inset a sketch illustrating the simplifiedarrangement of the radio cells of the two base stations;

FIG. 2 shows a graph similar to that of FIG. 1 with factors of timeaveraging and hysteresis introduced;

FIG. 3 is a diagram illustrating the inter-relationship of handoverparameters and handover qualities;

FIG . 4 shows a graph of received signal strength at a mobile station oftwo base stations (A, B) in a microcellular system, with inset asimplified plan of the two radio cells;

FIG. 5 is a schematic block diagram of apparatus forming part of thepreferred embodiment;

FIG. 6 shows a graph illustrating the response of the apparatus in FIG.5 in a microcellular environment; and

FIG. 7 is a table comparing typically examples of averaging delays andhysteresis margins for conventional systems and that of the preferredembodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

The base station and mobile station transceiver equipment for thecellular system are conventional and will not be described further. Themobile station has signal strength measuring equipment for measuring thereceived signal strength of base stations, which is likewiseconventional and will not be described further. The apparatus of thepreferred embodiment additional to these equipments is shown in FIG. 5and may be either at mobile or base station.

The additional apparatus of the preferred embodiment has inputs (I₀ toI_(N)) for receiving received signal strength data for the current cell(I₀) and adjacent cells (I₁ to I_(N)) to which the mobile stationobtaining the data may be handed over. Each input I is connected to along term averaging unit (LTA₀ -LTA_(N)) and a short term averaging unit(STA₀ -STA_(N)). The long term averaging unit (LTA₀) provided with thedata for the current communicating cell is connected to a first summerS_(N) which is provided with a narrow hysteresis margin HM_(N) while theshort term averaging unit STA₀ which is also provided with the data forthe current communicating cell is connected to a second summer S_(W)provided with a wide Hysteris Margin HM_(W). The first or long averagingperiod is several times the second or short averaging period. In apreferred embodiment, the first or long averaging period is between10-100 times the second or short averaging period, and most preferablythe first averaging period is substantially 20 times the secondaveraging period. The first hysteresis margin is a significant fractionof the second hysteresis margin. Preferably the first hysteresis marginis between 0.25 and 0.75 of the second hysteresis margin, and morepreferably substantially 0.5 of the second hysteresis margin. The outputof the first summer S_(N) is connected to a first comparator C₁ and theoutput of the second summer S_(W) is connected to a second comparatorC₂. The outputs of the remaining long term averaging units (LTA₁-LTA_(N)) are selective connectable via a first switch SW₁ to the firstcomparator C₁, and the remaining short term averaging units (STA₁-STA_(N)) are similar connectable via a switch SW₂ to the secondcomparator C₂. The outputs of the comparators C₁, C₂ are connected asinputs to an OR gate OR1, the output of which forms a Handover signal.

Typical values for the LTA=T, and STA=T₂, and HM_(N) =H₁ and HM_(W) =H₂ar shown in FIG. 7.

In a situation where pathloss varies gradually, e.g. a macrocell, thesystem operates generally as described above with LTA/HM_(N) circuitproviding for the current cell the significant comparison figure whichis compared in the first comparator C₁ with similar data for adjacentcells. When conditions for handover occur the first comparator C₁produces an out which via OR gate OR1, forms a Handover initiationsignal. On the other hand in situation where pathloss is a sudden, stepfunction e.g. the microcellular situation of FIG. 4 that STA/HM_(W)circuits operate to provide the signals necessary to determine handover.An example response based on the signal patterns of 4 is shown in FIG.6. Curve A shows the output of the second comparator C₂, while curve Bshows the output of the first comparator C₁. The curve A produces, (e.g.FIG. 4) an output forming a handover request with a 1 second delay afterthe sudden step function in the RSSI of the base station A (FIG. 4),while the curve B, i.e. comparator C₂, also produces a response but some6 seconds after the step function occurs. Thus in the, e.g.microcellular step function pathloss case the apparatus of the preferredembodiment provides the necessary handover on a time scale appropriateto the circumstances.

I claim:
 1. A cellular radio system comprising a plurality of stations,the plurality of stations including:a plurality of base stations eachhaving a radio transceiver serving a call of the system; a mobilestation having a radio transceiver for communicating with at least oneof the base stations; wherein at least one of the stations is a handoverdetermination station which obtains handover criterion measurements withrespect to at least two of the plurality of base stations for use indetermining which of the at least two base stations is the base stationwith which the mobile station should communicate; wherein the handoverdetermination station includes:a first means for time averaging thehandover criterion measurements over a first averaging period to obtainfirst time averaged handover criterion measurements, means for adding afirst hysteresis margin to the first time averaged handover criterionmeasurements to provide a first handover indicator; a second means fortime averaging the handover criterion measurements over a secondaveraging period to obtain second time averaged handover criterionmeasurements, said second averaging period is relatively short comparedto the first averaging period, means for adding a second hysteresismargin, which is relatively large compared to the first hysteresismargin, to the second time averaged handover criterion measurements toprovide a second handover indicator; and means for assessing handoverrequirements of the mobile station on the basis of the first and secondhandover indicators; wherein relatively gradual variations in thehandover criterion measurements are assessed and relatively suddenvariations in the handover criterion measurements are also assessed. 2.A cellular radio system as claimed in claim 1 wherein the handovercriterion measurements are selected from at least one of thefollowing:1) signal strength of the base station at the mobile station;2) signal strength of the mobile station at the base station, 3) biterror rate of the communication between the mobile station and the basestation, 4) level of interference of two base stations operating atsubstantially the same frequency, 5) relative distance measurement ofthe mobile station from two closest of the plurality of base stations.3. A cellular radio system as claimed in claim 1 wherein the firstaveraging period is several times the second averaging period.
 4. Acellular-radio system as claimed in claim 1, wherein the first averagingperiod is between 10-100 times the second averaging period.
 5. Acellular radio system as claimed in claim 1 wherein the first averagingperiod it substantially 20 times the second averaging period.
 6. Acellular radio system as claimed in claim 1 where the first hysteresismargin is a significant fraction of the second hysteresis margin.
 7. Acellular radio system as claimed in claim 1 where the first hysteresismargin is between 0.25 and 0.75 of the second hysteresis margin.
 8. Acellular radio system as claimed in claim 1 wherein the first hysteresismargin is substantially 0.5 of the second hysteresis margin.
 9. A methodof assessing handover requirements of a mobile station in a cellularradio system comprising a plurality of stations including the mobilestation and a plurality of base station, wherein each of the basestations has a radio transceiver serving a cell of the system and themobile stations has a radio transceiver for communicating with one orother of the base stations, and wherein at least one of the stationsobtains handover criterion measurements with respect to at least two ofthe plurality of base stations for use in determining which of the atleast two base stations is the base station with which the mobilestation should communicate; the method comprising the steps of:timeaveraging handover criterion measurements over a first averaging periodto obtain first time averaged handover criterion measurements, adding afirst hysteresis margin to the first time averaged handover criterionmeasurements to provide a first handover indicator; time averaginghandover criterion measurements over a second averaging period which isrelatively short compared to the first averaging period to obtain secondtime averaged handover criterion measurements, adding a secondhysteresis margin, which is relatively large compared to the firsthysteresis margin, to the second time averaged handover criterionmeasurements to provide a second handover indicator; and assessinghandover requirements of the mobile station on the basis of the firstand second handover indicators.
 10. A method according to claim 9 whichcomprises the steps of:averaging, over the first averaging period andover the second averaging period, signal strength measurements of atleast some of the base stations taken at the mobile station, to providea first averaged data over the longer averaging period and a secondaveraged data over the shorter averaging period; summing with the firstaveraged data, for a first selected one of the plurality of basestations, the first hysteresis margin and summing with the secondaveraged data, for the first selected one of the base stations, thesecond hysteresis margin, to provide a first and a second signalstrength indicator respectively; comparing the first signal strengthindicator with a signal strength measurement for at least a secondselected one of the base stations averaged over the first averagingperiod; comparing the second signal strength indicator with a signalstrength measurement for at least the second selected one of the basestations averaged over the second averaging period, and determining onthe basis of the comparisons the necessity for handover or cellre-selection for re-registration.